A time-of-check/time-of-use (TOCTOU) race condition in fastschema through v0.15.1 allows an unauthenticated remote attacker to bypass the OTP attempt limit on the account recovery flow, enabling brute-force attacks on 6-digit OTP codes. The verifyOTPSession function in pkg/auth/local.go reads and increments the attempt counter in separate non-atomic operations, allowing concurrent requests to observe the same attempt count below the threshold and proceed past the limit check before any update is committed.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
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